1. Field of the Invention
The present invention relates to a developing device, and a process cartridge and an image forming apparatus that use this developing device.
2. Description of the Related Art
In a conventional two-component developing device using a two-component developer consisting of magnetic carrier and toner, by using a developer stirring and transporting member that is disposed within a developer container storing the developer, the two-component developer is stirred, and after the toner concentration is evened out, thus obtained developer is transported toward a developer carrier having magnetic poles therein. In this manner, the developer carrier to which the developer is transported and supplied further transports this developer by carrying it on the surface of the developer carrier, supplies it to an electrostatic latent image formed on an image carrier, and thereby visualizes and develops the electrostatic latent image by means of the toner.
In this case, in the developer stirring and transporting member two screws are disposed parallel to each other such that their transporting directions are opposed to each other, wherein one of the screws supplies the developer to the developer carrier, while the other one draws up the developer stored in the developer container and delivers the stirred developer to the first screw, whereby the developer is circulated. In the course of this circulation, toner stirring is performed.
Furthermore, the developer stirring and transporting member recovers the developer that is used once for development to mix it with the supplied toner. If this mixing is not performed thoroughly, image density unevenness and the like occur, causing an abnormal image. The required stirring ability is determined by relative toner consumption with respect to the developer. The consumption of toner with respect to developer capacity increases when achieving high speed and size reduction, thus it is necessary to further improve the stirring ability.
In the conventional developing device, delivery of the developer between the two screws was not always performed smoothly. The reason is that the force of each screw for transporting the developer acts on the axial directions of the screws, and therefore there is basically no transportation force in the direction of delivery. For this reason, it is necessary to significantly increase the drive torque of the sending screw, but this imposes stress on the developer and thereby causes deterioration in adhesion and the like. Especially when the developer capacity is reduced due to size reduction or the like of the image forming apparatus, the number of times that the developer passes the delivery section between the screws relatively increases, thus the effect of development deterioration or the like becomes significant.
In order to improve such problems, there has been proposed an idea of increasing the screw diameter of an end portion of a screw, applying a force to the developer in a delivery direction, and then properly sending the developer so that decrease in torque and generation of stress in the developer are prevented (see Japanese Published Unexamined Patent Application No. H11-24404).
There is also proposed an idea of increasing the force applied at the time of delivery of the developer by regulating the angles of the screws, and thereby improving the flow of the developer in the delivery section so that the developer can be charged uniformly (see Japanese Published Unexamined Patent Application No. 2003-107859).
However, although these publications describe that the flow of the developer is improved by fulfilling the developer transportation force in the developer delivery direction by improving the screws, the problem is that such improvement of the screws is still not enough to alleviate the stress generated in the developer, thus the fluidity of the developer and uniform charging characteristics cannot be obtained.